Allemand was taken Monday night to University Hospital in New Orleans after a suicide attempt, the publicist, Penelope Jean Hayes, said in a written statement.
Anderson found the actress and called 911. Allemand was taken off life support Wednesday because of "a critical loss of brain and organ function, life support was withdrawn."
Hayes said Allemand died with her "mother, boyfriend and other life-long friends by her side."
In a statement provided to the AP by Anderson, he thanked family, friends and fans for "all the love and support."
"Gia was the most beautiful person I knew inside and out and she always smiled and made everyone else around her smile," Anderson said. "She had such an amazing impact on my life and anyone that knew her was blessed. All I have left is to cherish those memories we made together and help perpetuate the many values, faith and love that we shared."
Hayes did not provide further details about how Allemand died.
Allemand, a professional ballet dancer and actress, began modeling when as a baby she did Johnson & Johnson ads and also appeared as a Gerber baby. In addition, she did modeling work for Maxim magazine.
In her 2010 appearance on "The Bachelor," she was the third-runner up on the show featuring love interest Jake Pavelka. According to a statement from his publicist, Charmaine Blake, Pavelka offered well wishes to Allemand's family.
"We have lost an angel and a very dear friend," said Pavelka, who added that he and Allemand remained friends after the show's end.
Funeral arrangements are pending.